A fuel cell, for example, a solid polymer type is produced by composing unit cells each assembled by installing an anode and a cathode while sandwiching a solid polymer film between them and separators, and by stacking the unit cells in number of several hundreds. A fuel gas such as hydrogen or the like is supplied through a gas supply groove formed in one separator in anode side and an oxidizing gas such as oxygen or the like is supplied to the cathode side to cause electrochemical reaction to convert the chemical energy which the fuel has into the electric energy as output.
As a characteristic property as a material of the separators to be used for such a fuel cell, since electric current generated in each unit cell flows through the separators and respectively neighboring unit cells are assembled as to compose a structure in series connection in terms of a circuit by closely attaching the separators of the respective unit cells one another, the separators are required to have contact resistance as low as possible between surfaces of the neighboring separators and between the contact surfaces of the separators and the electrodes closely attached to the separators and are required to have the intrinsic resistance of the separators themselves (hereinafter referred also as to volume resistance) as low as possible as well.
Further, since the fuel gas and the oxidizing gas are supplied to the respective electrodes while being completely separated, gas impermeability in a high degree is required. Further, as described above, since a large number of unit cells are stuck to be assembled, the thickness of the separators is made as thin as possible and even if the separators are made thin as described, the separators are required to have sufficiently high mechanical strength and excellent molding precision as well from a viewpoint that fuel cells are assembled by stacking several hundreds of separators and fastening and fixing them.
As separators required to have such characteristic properties, well-known ones are isomg, for example, metal sheets of such as pure copper, a stainless steel and the like, however, in the case of such a metallic material, there is a problem that material deterioration is easy to be caused by hydrogen embrittlement owing to contact with hydrogen gas as a fuel gas and such a metal material is insufficient for a long time stability.
Therefore, those which have recently been developed are fuel cells employing a molded body produced by mixing a graphite powder with a thermosetting resin such as phenol resin as a binder and pressure molding the resulting mixture as separators. Since the graphite has a low electric resistance and excellent corrosion resistance, the above described problem in the case of using a metal can be improved. Further, since the void gaps formed in the inside of the compacted powder molded body are filled with the binder, gas impermeability to a certain extent can be obtained.
Such a separator made of graphite has conventionally been produced by, for example, using a resin-mixed graphite powder produced by steps of stirring thermosetting resin such as powder phenol resin with a volatile organic solvent such as an alcohol to obtain slurry, mixing and kneading a graphite powder with the slurry, drying the resulting mixture, and then pulverizing the dried mixture to a prescribed average particle diameter. In the above described pulverization step, the graphite powder whose surface is coated with the non-conductive resin by the kneading is pulverized and owing to that, produced is a raw material powder of graphite whose surface is exposed. Then, the raw material powder is filled in a prescribed molding die and pressure-molded to form a separator for a fuel cell.
In this case, the resin content is higher, the mechanical strength and the gas impermeability become more excellent. Consequently conventionally, a separator made of graphite has been produced by specifying at first the resin amount sufficient to satisfy the factors such as the mechanical strength and the gas impermeability necessary for a separator of a fuel cell.
However, a conventional separator made of graphite produced by the above described production method does not necessarily satisfy the electric characteristic properties such as volume resistance and the like. In other words, although the electric characteristic properties become more excellent as the resin amount is less, the resin amount cannot be decreased so much since the mechanical strength and the gas impermeability are decreased if the resin amount is decreased and for that, a conventional separator is not provided with excellent electric characteristic properties as well.